Electron-transfer (ET) reactions play a key role in many biological processes. In these systems the donor and acceptor sites can be separated by distances of 10-25 angstrom unit. Our aim is to probe the factors that control the rates of long-range ET: site- to-site distance, driving force, nature of the intervening protein medium and of the redox centers, and reorganization energy. We intend to study and to compare intramolecular long-range ET reactions in ruthenium (II/III) derivatives of structurally well- characterized iron and copper proteins: bovine cytochrome b5, Alcaligenes denitrificans azurin, and high-potential iron-sulfur proteins from Chromatium vinosum, Thiocapsa roseopersicina, and Chromatium gracile. We also intend to study mutants of rat cytochrome b5 with modifications designed to address medium and distance effects systematically. During the next three years, we propose: (1) to prepare highly purified samples of the above wild-type and mutant proteins labeled at selected surface histidine residues with ruthenium (II/III) reagents; (2) to characterize these modified proteins by spectroscopic and electrochemical measurements; (3) to determine by flash photolysis experiments the rates of ET between the protein redox center (iron-heme or other metal phorphyrin, blue copper, or iron-sulfur cluster) and the surface- bound ruthenium; and (4) to analyze the electronic (distance, medium) and energetic (driving force, reorganization energy, temperature) components of the long-range ET rates in these prototypal heme, blue cooper, and iron sulfur proteins. The proposed experiments will allow us to develop a better understanding of a fundamental process essential to all living organisms.